Torsion suspension system for motorcycles having improved center of gravity

ABSTRACT

A motorcycle having improved center of gravity is disclosed. The motorcycle has a torsion suspension system and a frame having front and rear wheels with a motor located in the frame and a driver&#39;s seat mounted on an upper supporting structure of the frame. A swing arm having a pivot point on one of the frame and the motor is provided, the swing arm having at least one arm supporting a wheel, the swing arm permitting the wheel to rotate about a generally horizontal axis and to move in a generally vertical direction with respect to the pivot point axis of said swing arm. A fuel tank is provided having at least a portion thereof fixedly mounted in said frame below said driver&#39;s seat, thereby improving said motorcycle center of gravity by providing for lowered weight.

This is a continuation-in-part of my U.S. Ser. No. 389,664, filed6-18-82, now U.S. No. 4,451,065.

This invention relates to suspension systems and more particularly itrelates to suspension systems for two and three wheeled vehicles such ascycles, tricycles and motorized versions of such vehicles.

In conventional suspension systems used for providing the rear wheel ofa motorcycle, for example, with up and down travel for purposes ofriding comfort and stability, hydraulic shock absorbers with helical orcoiled springs surrounding the shock absorber have been used andtraditionally have been mounted on a swing arm which carries the rearwheel, the placing of one end of the shock absorber on the swing armbeing in the vicinity of the rear wheel axle, the opposite end beingmounted or secured to the motorcycle frame. However, one of thedisadvantages of mounting the shock absorber on the swing arm close tothe axle is that increased wheel travel can normally only be gained withlonger shock absorbers. To offset this problem the spring/shock absorbercombination have been mounted away from the rear wheel axle towards theswing arm pivot point and thus the rear wheel could travel up and downwith the travel of the shock absorber only being required to be aportion of the rear wheel travel. However, even with the shock absorbersmounted in this position, the suspension still retained the disadvantageof having a relatively constant rate of compression; that is, the sameamount of force is required to compress the spring/shock absorber agiven distance at the start of the travel as required towards the end ofthe travel. This led to the use of single spring/shock absorbers mountedwith the use of linkage to require only light loads or small forces tocompress the shock absorber in the initial travel of the rear wheel andrequiring greatly increased loads towards the end of the rear wheeltravel. This system ensures that the rear wheel suspension would respondreadily to small bumps or undulations in the topography in its initialtravel but would not bottom on large bumps when greater forces aretransmitted to the spring/shock absorber. Such a suspension system isillustrated in U.S. Pat. No. 3,907,332. However, while this type ofsuspension system improves riding comfort and stability, it suffers fromthe disadvantage of earlier suspension systems by keeping weight of themotorcycle relatively high, normally above its center of gravity, andtotal weight of the suspension is increased because of the use of thelinkage. This detracts from stability with respect to balance; that is,top heaviness becomes somewhat more exaggerated as the distance of thisweight from the ground is increased by virtue of longer suspension, forexample. From the above, it will be appreciated that weight, in the formof suspension components e.g. linkage added to a motorcycle above itscenter of gravity, will detract from balance.

A further problem inherent in the design of the conventional shockabsorber/helical spring combination, regardless of mounting position, isthat tuning of the shock absorber and helical spring is not easilyseparable. That is, the compression of the spring and shock absorber iseffected at the same rate. Further, the shock absorber usually has to beconstructed with a certain rigidity in order to withstand the forcesresulting from the compression of the spring, all of which addsunnecessary weight.

The present invention resolves all of these disadvantages by providing asuspension system which in a motorcycle, for example, can put thesuspension weight below the center of gravity and permits the shockabsorber to be designed with respect to its use and not the use of aspring component. Additionally, these benefits can be obtained withoutcompromise with respect to the quality of ride whether the vehicle beused in competition where vigorous demands are made on the suspensioncomponents or whether the vehicle be used for touring or commuting wheredemands are relatively mild. Thus, it will be appreciated that thepresent invention, since it provides these benefits without compromiseor without adding undesirable high weight, results in a marked advancein the art of suspension systems, particularly with respect to two orthree-wheeled vehicles such as motorcycles.

SUMMARY OF THE INVENTION

An object of the invention is to provide a suspension system suitablefor use on bicycles such as motorcycles and motorized tricycles and thelike.

Another object of this invention is to provide a torsion bar suspensionfor bicycles and tricycles such as motorcycles and motorized tricycles.

Yet another object of this invention is to provide a torsion barsuspension having a rising or progressive force rate for motorcycles andmotorized tricycles.

And yet another object of this invention is to provide a suspensionsystem comprising at least one torsion bar and a separately tunablehydraulic shock absorber.

And still a further object of this invention is to provide a suspensionsystem comprising at least one torsion bar and linkage means connectedthereto and to the swing arm in a way which produces decreasing leverageand an apparent increase in the spring rate in the torsion bar inresponse to displacement of the wheel.

In accordance with these objects there is provided a suspension systemsuitable for bicycles, motorcycles and motorized tricycles. Thesuspension system includes a frame which may have a generally closedloop shape with a swing arm pivotally mounted or secured to the frame orto a motor carried in the frame. The swing arm has preferably two armsfor supporting the wheel in a generally horizontal axis, which permitthe wheel to rotate about the axis. Further, the swing arm permits thewheel to be displaced in a generally vertical direction and in an arcdefined by the swing arm. A torsion bar suspension member having a firstpart or first end portion and a second part or second end portion issecured to or carried by the frame at the first part. A lever arm havinga first end and a second end has its first end rigidly secured to thesecond portion of the torsion bar. A linkage means is provided andincludes a member which can be pivotally fastened to both the swing armand to the second end of the lever arm. The swing arm, linkage means,lever arm and torsion bar are connected such that movement of the swingarm displaces the linkage means thereby forcing the lever arm to twistthe torsion bar to provide a spring action. The linkage means and armmember are connected in a configuration or geometry which providesdecreasing leverage as the swing arm is deflected upwardly, resulting inan apparent increase in the torsion bar spring rate. In a preferredembodiment, a shock absorber is mounted on the linkage means so as toprovide an increase in the rate of damping in response to the wheeldisplacing or deflecting the swing arm in a generally upwardlydirection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a motorcycle schematicallyillustrating the use of the torsion suspension system and a portion of agas tank located under a seat in accordance with the present invention.

FIG. 1a is a side elevation view of a motorcycle schematicallyillustrating the use of the torsion suspension and a gas tank locatedunder a seat in accordance with the present invention.

FIG. 2 is a perspective view illustrating the torsion suspension systemand showing the mounting of the system on a motorcycle frame.

FIG. 3 is a view along the line III--III of FIG. 2.

FIG. 4 is a view of the suspension system similar to that of FIG. 3showing a shock absorber mounting position.

FIG. 5 is a perspective view illustrating another aspect of the torsionsuspension system in accordance with the present invention.

FIG. 6 is another perspective view showing a further embodiment of thepresent invention employing two torsion bars.

FIG. 7 is a view of the suspension system similar to that shown in FIG.6 showing a hydraulic shock absorber mounted so as to provide anincreasing or rising damping or rebound damping characteristic.

FIG. 8 is a perspective view illustrating an embodiment of the torsionsuspension system wherein the torsion bars are mounted in the swing arm.

FIG. 9 is an end view showing the torsion bars of FIG. 8 having a shockabsorber mounted on the linkage means so as to provide a progressiverate of damping.

FIG. 10 is a sectional view of a shock absorber body showing a dampingor metering system exterior to the shock body.

FIG. 11 is a detailed sectional view of the damping or metering systemof FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is shown a motorcycle, referred to generally as 20, andschematically illustrated thereon is a torsion suspension system 80 inaccordance with the present invention. The motorcycle includes a frame30 which, in FIG. 1, has the general configuration of a parellelogramhaving sides 31 and 32 and having a bottom side 33 and a top side 34which carries an air cleaning element 70. Frame 30 provides a cradle formotor 40. In addition, there is provided a juncture or steering headmounted generally where sides 31 and 34 are joined, the steering headproviding for attaching telescopic forks 50 which carry front wheel 52.A support member 32A is provided to carry seat 36. At the rear of theframe, swing arm 60 carries rear wheel 64 and is pivotally secured ormounted at point 61. This permits the rear wheel and swing arm to bedisplaced upwardly and downwardly with respect to frame 30 in responseto undulations or bumps or ruts in the topography or terrain over whichthe wheel may roll. Torsion rod or bar suspension system 80 is shownattached at about the juncture of side 33 and side 31 of the frame andto swing arm 60 and carried on a bearing member 88 (FIG. 2) to provideresiliency or spring action to the rear wheel as it rolls over terrain,the suspension system to be described in greater detail hereinbelow.That is, as the rear wheel rotates in swing arm 60, the swing arm andwheel as a unit are displaced or deflected generally upwardly ordownwardly in response to ruts or rocks or other uneveness encounteredon the terrain over whick the rear wheel rolls. The ability of the swingarm to move in this fashion adds greatly to riding comfort, particularlywhen the swing arm is sprung with respect to the frame and the properamount of spring damping is provided by a properly tuned hydraulic shockabsorber.

While the frame has been described hereinabove with respect to thegeneral shape of a parallelogram, it should be noted that for purposesof the present invention its application is not necessarily limitedthereto. That is, use of the present torsion suspension system onlyrequires mounting points. For this purpose, the motor may provide suchmounting points, particularly where the motor is used as a stressmember. Thus, it will be understood that the bottom of the motor, forexample, may provide mounting points at the forward end (the torsion barend away from the end connected to the swing arm) of the torsion bar andfurther the motor may provide for mounting or bearing points near wherethe torsion bar is connected to the swing arm. These mounting points canbe conveniently provided on the cases during fabrication or castingthereof. Further, while in FIG. 1 torsion bars of the suspension system80 are shown mounted substantially horizontally and in the direction inwhich the motorcycle travels, it should be understood that othermounting positions such as generally perpendicular, for example, withrespect to the swing arm point or any convenient angle betweenperpendicular and parallel to the swing arm pivot point. While it maynot be desired to mount a single torsion bar concentric with or parallelto the swing arm pivot axis because such mounting normally results invery short torsion bar lengths and therefor limited travel, angles at aslittle as 5° from the swing arm axis can be utilized since such anglepermits the torsion bar to be directed so as to provide for greaterlengths. Thus, it will be seen that the torsion bar mounting withrespect to the axis of the pivot point can be at an angle in the rangeof about 10° to 170° with preferred angles, as currently understood,being in the range of 60° to 120° with a typical angle being generallyabout 90° or substantially perpendicular to the axis of the swing armpivot point. For example, the torsion bar may be mounted generallyparallel to frame side 32, but this is a less preferred mountingposition since this can result in weight being added above the center ofgravity. Similarly, the torsion bar may be mounted on the swing arm orinside the legs of the swing arm for protection as will be explainedmore fully below. Further, combinations may be used, ie. one torsion barmay be mounted underneath as shown in FIG. 1 and one may be mountedgenerally vertically with respect to the swing arm pivot point. However,as presently understood the preferred mounting position is similar tothat shown in FIG. 1 because it lowers the center of gravity andconsequently increases stability and handling because of its low weightdistribution, particularly when compared to conventional systems where alarge part of the weight is above the center of gravity, which isnormally above the swing arm.

In addition to the above, it will be seen that struts or supports, e.g.for seat, may be mounted on the frame in the conventional manner withouthaving to be specially designed for the present torsion suspensionsystem. In recent developments in spring and shock absorbercombinations, air filter box 70 is mounted off center and carburetor 72is mounted on the engine barrel 74 at an angle, all of which greatlymakes servicing of such components extremely difficult. Further, sides32 of frame 30 normally have to be formed so as to provide greater spacefor the conventional spring/shock absorber and such forming or resultingbulk makes side member 32 protrude so as to interfere with the ridersstance, for example, when it becomes necessary to stand on the footpegs. Further, the mounting of the suspension system of the presentinvention has the advantage that frame members such as 32 and 34 may befabricated from considerably lighter weight material since they are notseverely stressed as in mounting of conventional spring/shock absorbercombinations which reduces weight of the motorcycle and againfacilitates balance and handling.

With respect to frame 30, it will be understood that twin sides areoften provided particularly with respect to bottom members 33 and sidemembers 32. However, side member 31 can be single component, usuallydepending on the manufacturer and as explained earlier, bottom member 33and a large portion of 31 can be removed if the motor is used as astress member.

Turning now to FIG. 2, there is illustrated one embodiment of torsionsuspension 80 of the present invention and a method of mounting such toa motorcycle frame (shown with broken lines) having twin sides andbottom members and to a swing arm 60 of the type having two parallelarms 62 which carry the rear wheel. The suspension system comprises atorsion rod or bar 82 having a first part thereof securely connected toframe member 84. Torsion bar 82 extends rearwardly therefrom throughframe member 86 wherein it is rotatably supported or carried by abearing 88. Torsion bar 82 has a second part 85 which extends past swingarm pivot point 61. However, it will be appreciated that torsion bar 82does not have to extend past the swing arm pivot point but may beshorter than the configuration which is shown in FIG. 2. An extensionmay be provided on the swing arm past its pivot point with a connectionbeing made to the torsion bar with similar results obtained as for theconfiguration shown in FIG. 2. An arm member or lever arm 90 has a firstend thereof securely fastened or connected to second part 85 of thetorsion rod and in the view shown in FIGS. 2 and 3 extends substantiallyhorizontally therefrom at about 90°. It will be understood that otherangles may be employed and further it will be understood that theconnection to second part 85 may be a spline or hex shape connectionwhich provides for ease of preloading the torsion bar.

Linkage means 94 pivotally connects second end 91 of arm member 90 toswing arm 60. That is, linkage means 94 is pivotally connected to boththe swing arm and second end 91 of the arm member. Arm member 90 may bea double member as shown or may have a double end to provide rigidity tothe system. Further, linkage means 94 may be directly pivotallyconnected to one arm of the swing arm or may be connected to one or twocross members connecting the arms of the swing arm. It should beunderstood that the linkage means is rotatably connected at either endand also should be mounted so as to permit the linkage means to pivotbecause the swing arm connection with the linkage means may be movedforward as the swing arm is deflected upwardly. In addition, linkagemeans 94 may be adjustable with respect to length in order to adjust ortune the amount of preload which may be desired in the suspensionsystem. Further, it will be appreciated that the dimensions of torsionbar 82, e.g. length and length of arm member 90 relate to softness ofharshness of ride for a given torsion bar, i.e. softness or stiffness ofthe torsion bar spring rate. Thus, with respect to arm member 90, it maybe provided with an adjustable length in order to easily adjust or tunethe spring for the type of terrain without removal from the frame orexchanging torsion bars.

Arm member 90 and linkage means 94 are important features of the presentinvention in that preferably they are connected to provide a rising ratewith respect to the spring rate of the torsion bar. That is, as the rearwheel swings the swing arm generally upwardly about its pivot point, theswing arm lifts linkage means 94 which in turn lifts arm member 90 whichtwists the torsion bar. As the swing arm lifts the linkage means, theeffective length of arm member 90 is decreased (see FIG. 3) whichprovides the rising spring rate, an apparent increase in the spring rateof the torsion bar. That is, as the swing arm is displaced upwardly bythe wheel, the leverage ratio on the torsion bar decreases. Thisprovides a suspension which is very soft at the initial travel toprovide a very smooth ride over small bumps. Yet when large bumps areencountered and the wheel deflects or displaces the swing armsubstantially, the effective spring rate of the torsion bar increasesand prevents the rear wheel from bottoming on the fender. Thus, it willbe seen that there is an apparent increase in the spring rate as thewheel is displaced upwardly.

For purposes of damping out oscillations set up by the torsionsuspension, a hydraulic shock absorber may be added as shown in FIG. 4,for example. One end of the shock may be mounted to arm member 92 andthe other end may be mounted to the swing arm in a manner similar tothat for linkage means 94. It will be appreciated that theprogressiveness of rate of damping in the shock absorber can be separatefrom that of the torsion bar and that the shock absorber can be mounteddepending on the linkage to have its rate of damping faster or slowerthan the torsion bar, Further, in this arrangement, unlike conventionalsuspension, the shock absorber can be much lighter and smaller since itdoes not have to carry a coiled spring as in most conventional systems.

In FIG. 5, there is shown an alternate embodiment, in which at least asecond torsion bar 182 is provided and mounted substantially parallel tofirst torsion bar 82. End 85 of torsion bar 82 is provided with the samearm member 90 and linkage means 94 and carried at end 85 on bearing 88on frame member 86 and additionally, torsion bar 82 is carried at theend 83 on bearing 87. Also, torsion bar 182 is mounted on frame member84 on a bearing and fastened or connected rigidly to frame member 86.Both torsion bars are connected to arm members 190A and 190B which arerotatably or pivotally connected by linkage means 194. This systemprovides the advantage of a longer torsion bar.

Alternately, both ends 83 and 183 of torsion bars 82 and 182 may berigidly mounted on frame member 84, with end 185 of rod 182 beingcarried in a bearing in frame member 86. Arm members 190A, 190B andlinkage 194 may be fastened on torsion bars 82 and 182 near frame member86. With torsion bars 82 and 182 being the same or different rates,additional progressiveness can be designed into the suspension system ofthe present invention. It will be understood that linkage 190A, 190B and194 may be set up so that torsion bar 182 only becomes at leastsubstantially effective when the swing arm has traversed 50% of itstravel, for example. Further, it will be understood that a longertorsion bar effect can be obtained by having concentric torsion bars.

Referring now to FIG. 6, there is shown a torsion suspension system inaccordance with the present invention wherein two torsion rods are usedwith two actuating systems. Thus, in this system there is providedtosion bar 82 which is carried by a bearing 88 and which is rigidlymounted or secured to frame member 84. An arm member 90 is mountedsecurely to torsion rod 82 at end 85 and linkage means 94 pivotally orrotatably mounted on arm member 94 with its other end adapted to berotatably mounted to the swing arm. A second torsion bar 282 is mountedgenerally parallel to torsion bar 82 as shown in FIG. 6. Torsion bar 282is securely connected to frame member 84 at end 283 and is carried onbearing 288 in frame member 286. At end 285 of torsion bar 282, armmember 290 is mounted securely and linkage means 294 is pivotally orrotatably mounted on arm member 290 at one end thereof. The other end oflinkage means 294 is adapted to be pivotally or rotatably mounted to theswing arm.

In the embodiment shown in FIG. 6, linkage means may be constructed soas to provide linkage means 94 and 294 in a generally parallel manner asshown. Or arm members may be mounted so as to produce a more progressiverising rate. That is, arm member 290 and linkage 294 may be arranged toprovide a greater amount of rising rate during the last portion of rearwheel travel than that of arm member 90 and linkage means 94.

In the embodiment shown in FIG. 6 arm member 190A and 190B may bemounted on torsion bars 82 and 282 substantially as shown in FIG. 6. Anadjustable member 194 can be provided to connect arm members 190A and190B. Adjustable member 194 can be adjusted so as to provide apre-loaded condition on the torsion bars.

In FIG. 7 there is shown a mounting position of a shock absorber 296which may be used with twin or double torsion bars 85 and 285 to providedamping which has a rising rate. Shock absorber 296 is provided withshaft 29 and 297 which are connected to arm members 90 and 290. Shafts29 and 297 may be connected to arm members 90 and 290 as shown; however,it will be evident that other mounting positions can be used, dependingto some extent on the amount of damping and the rate of damping orrebound damping desired. It will be understood that by rising dampingrate is meant that the rate with which shaft 297 travels or is movedinto the shock absorber body increases as the rear wheel is displacedupwardly. That is, the damping of the shock absorber can be arranged toincrease significantly in the last half of the wheel displacement ascompared to the first half of the wheel's displacement. Thus, it can beseen that with this arrangement the shock absorber can be set to bothaccomodate or provide a soft or smooth suspension in the initialdisplacement of the wheel and a much stiffer damping or rebound dampingin the last part of the wheel displacement. Shock absorber mountingpositions such as shown, for example, in FIG. 7, keep weight low whichgreatly facilitates stability and handling as noted hereinabove.

A shock absorber which can be used on the present invention may have abody, e.g. 296, with a cylindrical bore and a piston (not shown)connected to shaft 297. Shaft 29 may be firmly connected to body 296.Thus, in operation shaft 297 would slidably push the piston along thebore requiring hydraulic liquid or fluid contained in the shock to passthrough the piston and thereby dampen out oscillations in thesuspension.

In FIG. 8, there is illustrated the mounting of torsion bars 382 in arms62 of the swing arm 60 in accordance with the present suspension system.In this embodiment of the invention, end 383 of torsion bar 382,farthest away from swing arm pivot point 61 or closest to the rear wheelaxle, is fixedly secured to arm 62. At end portion 385 of torsion bar382 closest to the swing arm pivot point, the torsion bar is rotatablycarried in arm 62. Arm member 390 has a first end thereof which isrigidly attached to torsion bar 382 near end portion 385. Arm member 390extends from torsion bar 382 through an opening in the swing arm walltowards the second arm 62 and may extend generally parallel to the axisof the swing arm pivot point. Linkage means 394 pivotally connectssecond end 391 of arm member 390 to frame member 386 which is securelyattached to member 32. Thus it can be seen that linkage means 394 ispivotally connected to both arm member 390 and to frame member 386. Twotorsion bars may be attached to the frame essentially as described aboveand as shown in FIG. 8. Linkage means 394 and arm member 390 can bearranged so as to provide a rising or increasing spring rate in thetorsion bar as the rear wheel deflects the swing arm upwardly. It willbe understood that linkage means 394 and are member 390 may be connectedso as to provide varying degrees of increasing spring rate even thoughthe torsion bar is not changed. For example, the spring rate of thetorsion bar may be increased by shortening the length of arm member 390.Thus, it will be understood that many changes in the spring rate may bemade depending on the length of linkage means 394 and arm member 390.Likewise the spring rate of the suspension systems described hereinabovemay be varied accordingly.

The spring rate of any of the torsion bar systems herein described maybe set up so that the leverage ratio on the torsion bar can range from8:1 to 1.5:1.

In FIG. 9, it can be seen that a hydraulic shock absorber may be mountedso as to provide damping in the suspension systems. The shock absorbermay be mounted as shown in FIG. 9 so as to provide increasing dampingwhich can be substantially at the same rate of change as that of thespring rate of the torsion bar. However, since the shock absorber isessentially separate from the torsion bar, it may be set up so as toincrease or decrease damping with respect to the spring rate of thetorsion bar. Thus, it will be noted that, as compared to conventionalcoiled spring/shock absorber combination, the suspension systems of thepresent invention have the advantage of being able to increase ordecrease damping or rebound damping independent of the spring rate ofthe suspension system.

In FIG. 9, shock absorber 396 is shown attached to ends 391 of armmembers 390. It will be appreciated that other mounting positions forshock absorber 396 may be employed.

In the suspension system of the present invention, the amount of travelafforded the rear wheel can vary depending on the terrain over which themotorcycle travels. For example, if the motorcycle is of the type usedin competition such as motocross, the torsion suspension can be set toprovide as much as 13 or 14 inches of travel. If the use is for roadriding then the suspension can provide 5, 6 or 7 inches of travel, forexample.

For purposes of getting longer torsion bars, the torsion bars mounted asshown in FIGS. 8 and 9 may be connected by lever arms and linkage totorsion bars mounted under the motor, for example, providing aprogressive spring rate all of which are contemplated within the purviewof the present invention.

In another aspect of the present suspension system hereindescribed, itwill be understood that it permits redesign of the motorcycleparticularly for purposes of lowering or improving its center ofgravity. The lowered center of gravity makes for ease of handling andprovides significant benefits with respect to weight distribution. Inmotorcycles such as used in competition such as motocross, thesuspension, as noted earlier, is designed to have travel of about 12inches. This travel results in gas tanks being located a greaterdistance from the ground and inherently provides high weight which isundesirable. Conventional design of motorcycles do not readily permitrelocation of the gas tank. This is particularly true of the singleshock type motorcycle. Thus, there remains weight in the order of 20 to30 pounds particularly when the gas tank is full which adversely affectshandling characteristics and gives rise to an apparent increase inweight.

The present invention now permits redistribution of this high weight byrelocating a large portion of the gas tank 35 below the seat 36 as shownin FIGS. 1 and 1A. The gas tank may be shaped as shown in FIG. 1 toconveniently fit between frame members 32 and 32A. A gas cap 35A, havinga cover 36A may be provided for adding gasoline. A gasoline pump (notshown) of conventional design can be used to carry gasoline tocarburator 72. Air cleaner 70 can be located in the appropriate housingon top of frame member 34 and air carried therefrom to the carburatoralong conduit 70A. It will be appreciated that while carburator 72 isshown mounted directly behind cylinder 74 it may be mounted to the sidefor ease of access. If carburator 72 is mounted to the side of cylinder74 (FIG. 1A) then gas tank 35 may be extended further forward and closerto cylinder 74 as shown in FIG. 1A. Gas tank 35 can be suitably moldedaround carburator 72.

In FIG. 1A, carburator 72 is shown accessing the motor 40 through a sidemember in a manner conventionally used in two-stroke motors using arotary valve. Air to carburator 72 can be carried via tubing or framemember 32 from air chamber 70A. A gas cap 36A may be located on the sideof the gas tank (FIG. 1A) or on the top of the gas tank beneath airchamber 70A. Thus, it can be seen that having a large portion of gastank located underneath at least part of the seat greatly reduces thecenter of gravity of the motorcycle and greatly improves handling.

Thus having described preferred embodiments of the invention, it will beunderstood that certain parts may be changed or modified and that suchare contemplated to be within the scope of the invention. Further, whilethe invention has been described in terms of preferred embodiments, theclaims appended hereto are intended to encompass other embodiments whichfall within the spirit of the invention.

What is claimed is:
 1. A motorcycle having an improved center ofgravity, the motorcycle having:(a) a torsion bar suspension system; (b)a frame having a front and rear wheels with a motor located in saidframe and a driver's seat mounted on said frame; (c) a swing arm havinga pivot point on one of the frame and the motor, comprises at least onearm supporting a wheel, the swing arm permitting the wheel to rotateabout a generally horizontal axis and to move in a generally verticaldirection with respect to the pivot point axis of said swing arm; (d) atorsion bar suspension member having a first end portion and a secondend portion and mounted at an angle with respect to the pivot pointaxis, the first end portion secured to one of said frame and said motor,and said second end portion being rotatably carried by one of said frameand said motor; (e) a lever arm having a first end and a second end, thefirst end rigidly attached to said torsion bar; and (f) a linkage meanspivotally connected to said swing arm and to said second end of saidlever arm, said swing arm, linkage means, lever arm and torsion barconnected such that movement of the swing arm displaces said linkagemeans thereby forcing said lever arm to twist said torsion bar, thelinkage means and the lever arm connected so as to provide an apparentincrease in spring rate of the torsion bar as the wheel displaces theswing arm; and (g) a fuel tank having at least a portion thereof fixedlymounted in said frame below said driver's seat.
 2. The suspension systemin accordance with claim 1 wherein the torsion bar suspension member ismounted at an angle of at least 5° with respect to the axis of the pivotpoint.
 3. The suspension system in accordance with claim 1 wherein thetorsion bar suspension member is mounted at an angle between 60° and120° with respect to the axis of the pivot point.
 4. The suspensionsystem in accordance with claim 1 wherein the torsion bar suspensionmember is mounted generally perpendicular with respect to the axis ofthe pivot point.
 5. The suspension system in accordance with claim 1wherein the linkage means includes a member pivotally fastened to boththe swing arm and to the second end of the lever arm.
 6. The suspensionsystem in accordance with claim 1 wherein the torsion member is mountedunder said motor and extended generally perpendicularly towards the axisof the pivot point of the swing arm.
 7. The suspension system inaccordance with claim 1 wherein the swing arm has two arms two arms forsupporting said wheel and a cross-member is provided therebetween andsecured to each arm and said linkage means is pivotally connected tosaid cross-member.
 8. The suspension system in accordance with claim 1wherein the linkage means includes an adjustable linkage member forpurposes of changing the apparent increase in the spring rate of saidtorsion bar.
 9. The suspension system in accordance with claim 1 whereinthe length of the lower arm is adjustable for purposes of changing thespring rate of said torsion bar.
 10. The suspension system in accordancewith claim 1 including a hydraulic shock absorber having one end thereofconnected to said linkage means and having an apposed end attached tosaid frame.
 11. The suspension system in accordance with claim 1including a hydraulic shock absorber having an end thereof connected tosaid swing arm and having an opposed end connected to said frame. 12.The suspension system in accordance with claim 1 including a hydraulicshock absorber and linkage means connecting the shock absorber to theswing arm, the shock absorber responsive to the generally up and downmovement of the swing arm as it is deflected by the wheel.
 13. Thesuspension system in accordance with claim 1 including a hydraulic shockabsorber and linkage means connecting the shock absorber to the frameand the swing arm, the linkage means connecting said swing arm so as tochange the rate of damping in said shock absorber as the swing arm isdeflected upwardly.
 14. A motorcycle having an improved center ofgravity, the motorcycle having:(a) a torsion bar suspension system; (b)a frame having a front and rear wheel with a motor located in said frameand a driver's seat mounted on said frame; (c) a swing arm having apivot point on one of the frame and the motor, comprises at least onearm supporting a wheel, the swing arm permitting the wheel to rotateabout a generally horizontal axis and to move in a generally verticaldirection with respect to the pivot point axis of said swing arm; (d) atorsion bar suspension member having a first end portion and a secondend portion and mounted at an angle with respect to the pivot pointaxis, the first end portion secured to one of said frame and said motor,and said second end portion being rotatably carried by one of said frameand said motor; (e) a lever arm having a first end and second end, thefirst end rigidly attached to said torsion bar; and (f) a linkage meanspivotally connected to said swing arm to said second end of said leverarm, said swing arm, linkage means, lever arm and torsion bar connectedsuch that movement of the swing arm displaces said linkage means therebyforcing said lever arm to twist said torsion bar, the linkage means andthe lever arm connected so as to provide an apparent increase in springrate of the torsion bar as the wheel displaces the swing arm; and (g) ashock absorber mounted on said linkage means so as to provide anincreased rate of damping in response to the wheel displacing the swingarm. (h) a fuel tank having at least a portion thereof fixedly mountedin said frame below said driver's seat.
 15. The suspension system inaccordance with claim 14 wherein the torsion bar suspension member ismounted at an angle in the range of 60° to 120° with respect to the axisof the pivot point.
 16. The suspension system in accordance with claim14 wherein the torsion bar suspension member is mounted generallyperpendicular with respect to the axis of the pivot point.
 17. Thesuspension system in accordance with claim 14 wherein the torsion barsuspension member is mounted generally parallel of the axis of the swingarm pivot point.
 18. In a motorcycle having an improved center ofgravity,(a) a torsion suspension system; (b) a frame having front andrear wheels with a motor located in said frame and a driver's seatmounted on an upper supporting structure of said frame; (c) a swing armhaving a pivot point on one of the frame and the motor is provided, theswing arm having at least one arm supporting a wheel, the swing armpermitting the wheel to rotate about a generally horizontal axis and tomove in a generally vertical direction with respect to the pivot pointaxis of said swing arm; and (d) a fuel tank is provided having at leasta portion thereof fixedly mounted in said frame below said driver'sseat, thereby improving said motorcycle center of gravity by providingfor lowered weight.